The present invention relates to a tunneling magnetoresistive element, and a magnetic sensor using the same.
A remarkable magnetoresistance effect is caused in a small double junction system, and a tunneling magnetoresistive element (TMR) of the small double junction type is proposed according to a known example 1 (K. Ono, Hiroshi Simada, and Youiti Ootuka, "Journal of the Physical Valve Effect and Magneto-Coulomb Oscillations," Journal of the Physical Society of Japan, vol. 66, no. 5, May 1997, pp. 1261-1264).
FIGS. 12A and 12B show the conventional tunneling magnetoresistive element of the known example 1. FIG. 12A is a plan view, and FIG. 12B is a cross-sectional front view. As shown in FIGS. 12A and 12B, soft magnetic layers (electrodes) 200, 201 of Ni are formed on the surface of an insulating layer 10. In addition, tunnel barrier layers (oxide film barriers) 300, 301 are deposited on the soft magnetic layers 200, 201, respectively. A ferromagnetic layer (island) 100 of Co is also formed between the soft magnetic layers 200, 201.
When a voltage V is applied between the soft magnetic layers 200, 201 of this tunneling magnetoresistive element, electrons are injected into one soft magnetic layer 200, tunneling the tunnel barrier layer 300. Thus, a current path is formed which reaches the other soft magnetic layer 201 through the ferromagnetic layer 100 and tunnel barrier layer 301. The current along this path of electrons is called tunneling current I. This system is a double junction system since it includes two tunnel barrier layers 300, 301.
There is another known example 2 (D. V. Averin and Yu. V. Nazarov, "Single Charge Tunneling-Coulomb Blockade Phenomena in Nanostructures," ed. Hermann Grabert and Michel H. Devoret, Plenum Press, New York, 1992, Chap. 6, pp. 217-247). As described in this example, when the conductance property of the small double junction system of this element is measured with the bias voltage V fixed to about zero, the classic tunneling current I does not appear due to Coulomb blockade effect. If the voltage region in which the classic tunneling current I does not appear is called blockade region, the classic tunneling current I exists out of the blockade region, and is substantially proportional to the voltage V. In other words, when the bias voltage V is smaller than a voltage Vc (V&lt;Vc) which separates the blockade region from the outside, or when it is within the blockage region, the I-V characteristic of the small double junction can be expressed by the following equation. EQU I.varies.0 (1)
In addition, when the bias voltage V is larger than the voltage Vc (V&gt;Vc), or when it is out of the blockade region, the I-V characteristic of the small double junction can be expressed by the following equation. EQU I.varies.V/(R1+R2) (2)
Where R1, R2 is the tunnel resistance.
Even within the blockade region, a tunneling current due to the higher-order term from the viewpoint of quantum mechanics is observed, and this tunneling current I can be expressed by the following equation. EQU I.varies.{e.sup.2 V.sup.3 +(2.pi.kT).sup.2 V}/(R1.times.R2) (3)
Where e is the electric charge of an electron, and k is the Boltzmann constant.
This tunneling current I is also called current due to co-tunneling. As will be seen from the above equation (3), if the voltage V is constant, the tunneling current I is reversely proportional to the product of the tunnel resistances R1, R2. In addition, the tunneling current I includes the term proportional to the voltage V and the term proportional to the cube of the voltage V.
The current I due to co-tunneling effect is always present within and out of the blockade region. However, out of the blockade region, the current due to co-tunneling effect is negligibly small as compared with the classic tunneling current. Within the blockade region, since the classic tunneling current associated with the 0-order term is zero (suppressed), the tunneling current due to co-tunneling effect associated with high-order terms is chiefly observed.
The tunnel resistances R1, R2 change depending on the direction in which the magnetization of the ferromagnetic layer 100 is oriented with respect to the magnetization of the soft magnetic layers 200, 201. In other words, the conduction electron spin within the soft magnetic layers 200, 201 of Ni is affected even by an external weak magnetic field, and thus the spin direction is easily changed. The ferromagnetic layer 100 of Co does not easily follow the external weak magnetic field. As a result, the external magnetic field acts to switch the case in which the magnetization of electrons within the soft magnetic layers 200, 201 and the magnetization within the ferromagnetic layer 100 are parallel and the case in which those are antiparallel to each other. Consequently, the tunneling rate in the path from the soft magnetic layer 200, 201 to the ferromagnetic layer 100 or from the ferromagnetic layer to the soft magnetic layer 200, 201 is changed, so that the tunnel resistances R1, R2 are changed by the variation of the external magnetic field. The effect that the tunnel resistances R1, R2 are changed by the external magnetic field is called tunneling magnetoresistance effect. In the tunneling magnetoresistive element shown in FIGS. 12A and 12B, a change of tunneling current due to the tunneling magnetoresistance effect is observed when the external magnetic field is changed.
According to the above-given equations (1) to (3), the tunneling current I due to the co-tunneling effect, which is observed within the blockade region, is reversely proportional to the product of the tunnel resistances R1 and R2, but the classic tunneling current I observed out of the blockade region is only reversely proportional to the sum of the resistances R1 and R2. Accordingly, when the bias voltage V is constant, the tunneling current within the blockade region is more changed by the variation of the external magnetic field than that out of the blockade region. In other words, the change of the resistance R of the whole small double junction specified by the ratio of the voltage V and tunneling current I due to the variation of the magnetic field is more increased, or enhanced within the blockade region. That is, the change of the individual tunnel resistance R1, R2 within the blockade is the same as out of the blockade, while the resistance R of the whole small double junction is observed to be changed, making the current due to the co-tunneling effect be more changed. This effect can be ascribed to the high-order terms of the tunneling phenomenon. Particularly, this effect is formed by a mechanism different from the tunneling magnetoresistance effect that is implicitly associated only with 0-order term. However, since this phenomenon due to the co-tunneling effect is due to part of the whole tunneling phenomenon, this phenomenon is here called the enhancement of magnetoresistance effect based on the co-tunneling effect.
Since the tunneling magnetoresistive element of small double junction type shown in FIGS. 12A and 12B has its magnetoresistance effect resulting from the co-tunneling effect, it must be effectively operated as a single electron device. However, according to the known example 1, the tunneling magnetoresistive element was only operated at a very low temperature of about 20 mK. Moreover, according to the known example 1, the size of the ferromagnetic layer 100 is 150 nm.times.2500 nm. The size of the ferromagnetic layer 100 is required to be 5.times.5 nm or below in order to operate at room temperature. Thus, since the request for the operation at room temperature is very difficult to be accepted by the prior art, it is necessary to greatly reduce the size of ferromagnetic layer 100 by a fine working process which is impossible in the prior art.
In addition, the tunneling magnetoresistive element of small double junction type has an impedance higher than the conventional magnetoresistance (MR) effect element and giant magnetoresistance (GMR) effect element. The reason is that the tunnel resistance is required to be much larger than the quantum resistance RK (about 25.8.OMEGA.) in order to draw the Coulomb blockade effect. Thus, it is necessary that the tunneling magnetoresistive element be avoided from having the high impedance. If the tunneling magnetoresistive element cannot be fully avoided from the high impedance, a signal detection method different from the conventional one is necessary to use in order to improve the S/N ratio.